The use of multilayer films comprising multiple alternating layers of two or more polymers to reflect light is known and is described, for example, in U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.), U.S. Pat. No. 5,103,337 (Schrenk et al.), WO 96/19347, and WO 95/17303. The reflection and transmission spectra of a particular multilayer film depends primarily on the optical thickness of the individual layers. Optical thickness is defined as the product of the actual thickness of a layer and its refractive index. Accordingly, films can be designed to reflect infrared, visible or ultraviolet wavelengths .lambda..sub.M of light by appropriate choice of optical thickness of the layers in accordance with the following formula: EQU .lambda..sub.M =(2/M)*D.sub.r (Formula I)
wherein M is an integer representing the order of the reflected light, and D.sub.r is the optical thickness of an optical repeating unit (also called multilayer stack) comprising two or more polymeric layers. Accordingly, D.sub.r is the sum of the optical thicknesses of the individual polymer layers that make up the optical repeating unit. By varying the optical thickness of an optical repeating unit along the thickness of the multilayer film, a multilayer film can be designed that reflects light over a wide range of wavelengths.
From Formula I, it can also be seen that a multilayer film or optical body which is designed to reflect light in a first region of the spectrum may have higher order reflections in a second region of the spectrum. For example, a multilayer film designed to reflect infrared light will also have higher order reflections in the visible region of the spectrum. Specifically, a multilayer film designed to have a first order reflection (M=1) at 1500 nm may have higher order reflections at about 750 nm (M=2), 500 nm (M=3), 375 nm (M=4), etc. A film designed to reflect infrared light of even longer wavelengths may have even more higher order reflections in the visible region. Thus, for example, a multilayer film having a first order reflection at 2000 nm will have higher order reflections at 1000 nm, 666 nm, 500 nm, 400 nm, etc. These higher order reflections are undesirable in many applications (e.g., window films) because they impart an iridescent appearance to the film where a transparent, colorless appearance is preferred. Therefore, in order to design a multilayer film that reflects light over a first region of the spectrum (e.g., the infrared region) but does not reflect light over a shorter wavelength region (e.g., the visible region), at least two, and preferably at least three higher order reflections need to be suppressed.
U.S. Pat. No. 5,103,337 (Schrenk et al.) teaches that an infrared reflecting multilayer film having an optical repeating unit with polymeric layers A, B and C arranged in an order ABC, is capable of suppressing at least two successive higher order reflections when the index of refraction of polymeric layer B is chosen to be intermediate to that of polymeric layers A and C. In a particular embodiment of the film described therein, the optical repeating unit is formed by arranging layers A, B and C in an ABCB pattern. By selecting polymeric materials A, B and C such that the refractive index of material B equals the square root of the product of the refractive index of materials A and C, and by setting the optical thickness ratio for material A and C to 1/3 and that of material B to 1/6, at least three higher order reflections can be suppressed. Similar teachings are found in Thelen, A., J. Opt. Soc. Am. 53, 1266 (1963). However, one disadvantage of this design is that the amount of reflection of incident light with the first order harmonic decreases with increasing angle of incidence. A further disadvantage of this design is that the suppression of the three higher order reflections also decreases with increasing angle of incidence. This later result is particularly undesirable in applications such as window films where the infrared reflective film is used to shield a room from infrared sunlight, since the sunlight will frequently be incident at angles substantially away from the normal (particularly in the spring and summer when the sun is high in the sky).
U.S. Pat. No. 5,540,978 (Schrenk) teaches a multilayer polymeric film that reflects ultraviolet light. In one embodiment, the film includes first, second, and third diverse polymeric materials arranged in a repeating unit ABCB. In another embodiment, the layers are arranged in the repeating unit ABC.
WO 96/19346 teaches reflective films that are made out of an optical repeating unit of alternating layers A and B, where A is a birefringent polymeric layer and B can be either isotropic or birefringent. The reference notes that, by matching the index of refraction between both layers along an axis that is perpendicular to the surface of the film, the dependency of reflection on angle of incidence can be greatly reduced. However, the reference does not teach how these results can be extended to multilayer optical systems having three or more layer types in the repeating unit (e.g., films with ABC or ABCB repeating units). Such a system would be highly desirably, both because of the improvement in reflectivity at oblique angles it would afford, and because the additional layer or layers in the repeating unit could be used to impart better mechanical properties to the system. For example, one of the additional layers could be an optical adhesive that would reduce the tendency of the other layers to delaminate. Furthermore, while WO 96/19346 mentions infrared reflective films, it does not describe how an IR film can be made that will not suffer from higher order reflections in the visible region of the spectrum (e.g., if the first order reflection is at 1200 nm or more).
There is thus a need in the art for a multilayer film or other optical body that exhibits a first order reflection band for at least one polarization of electromagnetic radiation in a first region of the spectrum (e.g., in the infrared, visible or ultraviolet regions of the spectrum) but can be designed to suppress at least the second, and preferably also at least the third, higher order harmonics of the first reflection band. In particular, there is a need in the art for a multilayer film or optical body that has a first reflection band in the infrared region of the spectrum but that exhibits essentially no higher order reflection peaks in the visible region of the spectrum.
There is also a need in the art for a film or other optical body having three or more layer types in its optical repeating unit, and for which the reflectivity of the film (e.g., toward infrared radiation) remains essentially constant, or increases, at non-normal angles of incidence.
These and other needs are met by the films and optical bodies of the present invention, as hereinafter described.